Three National Weather Service teams spent Friday surveying storm damage in seven counties. But their ultimate goal is to save your life.

"We look at the damage to evaluate how we can do a better job next time," said John De Block, warning coordination meteorologist for the National Weather Service in Calera.

"With the fact that so many people died, we want to find out how to get people to respond better," he said. "In an EF4, EF5 tornado, you can't get complacent. Your only chance to survive is to get to a safe place."

At least 319 people died in seven states Wednesday -- 250 of them in Alabama -- a total that recalls 1974 or 1932, when waves of savage tornadoes struck with little warning.

This time people had a lot of warning.

For all the tornadoes in the Southeast on Wednesday, the average warning time was 24 minutes. The Alabama counties covered by the Calera weather service office had the same average warning time, and few of the warnings were false-positives.

Days before this deadly outbreak, the weather service had known of the approaching threat -- through analysis of national weather patterns -- and had widely spread the news.

"On Friday (April 22) we contacted emergency managers," De Block said. "Over the weekend we started to highlight the risks. On Monday we issued tornado safety rules to try to keep people safe. You need to know the rules ahead of time -- you can't learn the rules when the storm hits."

All this preparation and timely warnings are as good as present knowledge and technology can yield, yet still hundreds died.

"Our hope is that by doing damage assessment, we can improve our warning and increase the public response to our warning," De Block said.

De Block said his surveyors saw terrible damage when they surveyed Marion County on Thursday. In the small town of Hackleburg, a tornado traveling 66 mph killed at least 25 people in just 23 minutes. A Wrangler Jeans factory was left in rubble and bark was stripped off trees by the wind.

The ability to warn of imminent tornadoes has greatly improved in the past 15 years, largely thanks to Doppler radars and continued improvements in software and the algorithms used to analyze data. A Doppler radar measures how fast raindrops pushed by the wind move away from or toward by measuring their radar frequency shift, similar to the way the whistle of a train drops in pitch as it passes by. An area where half is moving away and half is moving toward is a sign of a rotating air mass that could spawn a tornado.

"The average warning time was below 10 minutes for tornadoes before Doppler," De Block said. "Now it's approaching an average of 15 minutes across the country."

The first signs of danger for Jefferson County residents on Wednesday came early.

Don Roybal, EMA officer for the Jefferson County Emergency Management Agency, was on duty at 2:26 a.m. when the National Weather Service issued its tornado watch.

"The first tornado warning went off a little after 5 a.m.," Roybal said. Jefferson County immediately sounded its 253 tornado sirens for 90 seconds. Each siren can be heard a mile, or a mile-and-a-quarter, away. Most of them rotate, spewing sound of the threat in all directions.

But the squall line, with 100 mph gusts that hit just before dawn on Wednesday, may have created a problem.

"Perhaps the biggest challenge was the morning storm, which caused widespread power outages," De Block said. "That may have prevented people from getting information later in the day (when the tornado hit). Or did they think that was it? We don't know."

De Block urges multiple sources of weather information -- sirens, a weather radio, television and radio, and the free cell phone warnings that are available from many companies.

"If you depend on just one source of weather information, it could be a big mistake," he said.

Getting the warning is only half of the solution.

"Ultimately, it comes down to personal responsibility," De Block said. "You have to make the decision to be prepared."

"You've got to decide what action you are going to take when the siren sounds the next time," he said. "You have to make the right choice."

De Block has seen improvement in tornado warnings first-hand.

In 1994, he was a young Air Force captain at Maxwell Air Force Base, in charge of implementing the first Doppler weather radar in Alabama and only the 11th in the United States.

When the Palm Sunday killer tornado hit Goshen Methodist Church near Piedmont that spring, "we recognized it as a tornado all the way from the Macon-Tallapoosa County line. That was the first use (of Doppler radar) in Alabama."

His team quickly shared the information with the weather service in Birmingham, but 20 people died in the fierce storm.

More improvements in tornado warning are close at hand for Jefferson County and central Alabama.

This year the Jefferson County sirens will be programmed to wail only if they are within the tornado warning polygon issued by the National Weather Service, Roybal said. Currently they sound all over the county, alerting people outside of the warning polygon and creating the risk of a "cry wolf" syndrome.

For people across central Alabama, new radar technology will be in place by late 2012. The National Weather Service is upgrading its 169 Doppler weather radars to "dual polarization" capability. In this system, two alternating radar beams are sent out -- one oscillating in the horizontal plane and the other in the vertical plane.

Setting aside all the complex technical details, this means meteorologists can better see tornado debris in the sky -- even leaves or grass -- if a tornado has touched down. They can also distinguish hail from rain, or rain from snow. The "dual-pol" radar will also improve flash flood warnings, because it will more accurately measure rainfall.